The present invention relates to photolithographic processing.
In order to create faster and more powerful integrated circuits, circuit designers are increasing the number and decreasing the size of circuit elements that are placed on an integrated circuit. With conventional photolithography, the minimum size of an object that can be created on a silicon wafer depends directly on the wavelength of light used to expose the wafer and inversely on the numerical aperture of the lens through which the light that exposes the wafer is passed. Because the costs associated with decreasing the illumination wavelength or increasing the numerical aperture can be prohibitive, chip manufacturers are continually looking for techniques that can create smaller objects on a wafer using existing photolithographic equipment.
One of the most powerful techniques for increasing the density of an integrated circuit with existing photolithographic equipment is with the use of phase shifters. As discovered by Marc Levenson of IBM and others, the phase of the light that strikes a wafer can be manipulated to destructively interfere at desired locations on the wafer in order to enhance image contrast and reduce diffraction effects that occur when the light passes through a pattern of opaque areas on a semiconductor mask. In addition, by selectively placing phase shifters on the mask, subwavelength features can be created on the wafer to form circuit elements.
While the use of phase shifting structures on a photolithographic mask allows increased contrast and the creation of subwavelength features using existing photolithographic equipment, the phase shifters are relatively expensive to create and may introduce errors into the mask. Therefore, there is a need for a method and apparatus for optimizing the creation of phase shifters on a mask that minimizes the possibility of errors and facilitates the production of the mask.
A method and apparatus for optimizing data used in the creation of a photolithographic mask. The apparatus includes a computer system for reading data that describes a desired physical layer of an integrated circuit. The computer creates multiple data layers and groups data structures that define different areas on the mask into one of the multiple data layers. After the data structures are defined and grouped into one of the data layers, the data structures grouped in one or more of the data layers are analyzed according to one or more design rules. One or more properties of the data structures are then assigned in accordance with the analysis performed.
In a currently preferred embodiment of the invention, the invention is used to optimize data that defines phase shifters on a photolithographic mask. The computer system creates data structures that define polygons that correspond to circuit elements and polygons that correspond to phase shifters on the mask. Polygons are then placed in one of the corresponding data layers. All the polygons associated with circuit features are grouped in one data layer and the polygons associated with distinct phase shift areas are grouped in separate data layers. The phase shifting polygons in the data layers are analyzed and a phase shift amount is assigned to each of the polygons after the polygons have been created, and based on the design rule analysis. In one embodiment of the invention, the design rules operate to minimize an etched area of the mask.
The present invention is not limited to the optimization of data used to create masks for integrated circuits. The present invention can also be used to optimize data used to create micro electromechanical structures (MEMS), thin film heads for disk drives or other devices created with photolithographic processes.